Wireless communication systems support various access technologies, each of which can take numerous forms and be implemented in different frequency bands. For example, the standard analog access technology for U.S. cellular applications is advanced mobile phone system (AMPS), which uses a range of frequencies between 824 megahertz (MHz) and 894 MHz. Digital cellular applications typically use frequency division multiple access (FDMA), time division multiple access (TDMA), or code division multiple access (CDMA) access technologies. FDMA systems typically operate between 824 and 893 MHz. TDMA systems normally operate at either 800 MHz (interim standard (IS)-54) or 1900 MHz (IS-136) frequency bands. TDMA technology is implemented in the global system for mobile communications (GSM) standard in three different frequency bands, depending on geography. For example, GSM operates in the 900 MHz and 1800 MHz bands in Europe (EGSM) and Asia, and in the 1900 MHz band in the United States. In addition to GSM, TDMA is used in PCS (personal communication services) based systems operating in the 1850 MHz to 1990 MHz bands. CDMA systems typically operate in either the 800 MHz or 1900 MHz frequency bands.
Given the lack of standardization and the varying infrastructure for the above systems, mobile terminals, such as mobile telephones, personal digital assistants, wireless modems, and the like, often need to communicate in different bands and operate in different modes, depending on the type of transmission technology used. In addition to providing analog capability, newer phones are supporting multiple modes and frequency bands.
To support such operation, mobile terminals typically incorporate dedicated low-noise amplifiers (LNAs) and associated filtering for each frequency band supported. Further, the down-conversion circuitry is configured to down-convert the received signals from various frequency bands, using various transmission technologies, to an intermediate frequency (IF), or directly to a baseband level for baseband processing. The mixers used in the down-conversion circuitry are driven by local oscillators (LOs) having varying frequencies depending on the mode of operation.
Unfortunately, the local oscillator energy can leak into the system and cause a DC offset in the down-converted signal. In some configurations, the local oscillator energy may leak into the antenna, which results in a signal that is amplified by the LNA and mixed into the down-converted signal, resulting in a DC offset. The local oscillator energy may also leak into the LNA inputs, causing the same result. Further, the local oscillator energy may leak into the input of the mixer, and ultimately be mixed with itself to create a DC offset. DC offsets may also be caused by mismatched devices that create an imbalance among differential signals, out-of-band signals from other mobile terminals, out-of-band signals from base stations, and the like.
The DC offset is particularly detrimental in systems wherein the down-converted signals are represented as baseband signals. Even in the absence of leakage signals, the differential signals provided by the down-conversion circuitry should have a common DC level. Typically, DC correction circuitry is used to sample the down-converted differential signals and adjust their DC levels to minimize the impact of any DC offset during baseband processing. For multiple band and multiple mode mobile terminals, switching from one mode or band to another typically affects the DC offsets associated with the differential signals, and requires a DC adjustment of these signals prior to receiving signals in each mode.
During DC offset correction, the antenna must be blocked so that the DC correction circuitry does not falsely lock onto an instantaneous signal level associated with a desired or interfering receive signal. Traditionally, there have been at least two methods to isolate the antenna from the rest of the system. A first method is to use a transmit/receive switch or duplexer to open the path between the LNA and the antenna, and thus avoid any signals being presented to the LNA inputs. This method has proven unacceptable, in that the switch or the duplexer cannot provide complete isolation, and thus allows signals appearing at the antenna to reach the LNA. This results in an almost random DC level at the outputs of the LNAs. A second method is to turn off the LNA, and thus block signals from reaching the down-conversion circuitry. In this method, where the LNA is deactivated, the DC offset correction takes place without compensating for LO leakage. Thus, when the LNA is reactivated, the DC offset caused by leakage signals is present. In essence, the latter method corrects for circuit imperfections, but does not address DC offsets induced by leakage signals. Accordingly, there is a need for an improved architecture and process for addressing and correcting DC offset due to signal leakage and the like.
The assignee of the present invention is also the assignee of U.S. patent application Ser. No. 10/072,361, filed 7 Feb. 2002, now U.S. Pat. No. 6,816,718, which is hereby incorporated by reference in its entirety. The '361 application details one technique to address the DC offset problem using a dummy LNA. However, the use of the dummy LNA may not be optimal in certain situations. Thus, there is a need for an alternate solution.